In view of very disparate relaxation times measured in experiments on wide GaAs/AlxGa1-xAs quantum wells, we have calculated the electron-LO-phonon intersubband scattering rate with a thermal distribution of electrons in the quantum well subbands. The intersubband transition can proceed through LO-phonon emission even at wide well widths through the high energy tail of the thermal distribution. Our results show that at low electron temperature the scattering rate has a very sensitive dependence on electron temperature, resulting in a wide range of lifetimes, from picosecond to over a nanosecond. This sensitivity of the scattering rate to electron temperature makes it possible to account for the large variation in decay times that have been measured in similar wide wells. We find that at room temperature the lifetimes are around 1 ps at all well widths. Detailed results are given of the dependence of the scattering rate on temperature and carrier concentration. © 1995 The American Physical Society.